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Introduction to Linux - A Hands on Guide

This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.

few weeks ago I met some problem I can't solve myself. To be short, my home network consists of two routers running openwrt, they are located in two different flats, connected by one of city network providers. So, those routers have the following vpn addresses: 10.10.138.37 and 10.0.5.150. There is no problem in connectivity between those 2 devices, all is sent from one is for sure will be received by other. But in order to get rid of NAT between devices in 2 flats, I decided to create vpn tunnel between my devices. Inside my flats I have two subnetworks: 192.168.1.0/24 and 192.168.2.0/24. The simplest way to connect them is GRE tunnel which requires only gre encapsulation support and ip utility. Further I provide my configuration (router1 is rt53, router2 is rt109):

packet loss happens only inside tun0 iface! That's very strange... Before some moment there was no such loss... I think it could happen after I swapped into new release of openwrt, but nevertheless, I want to understand where is the core of the problem lies!

I did tcpdump'ing of icmp packets from both sides and saw, that packets sent from rt53 to rt109 via tun0 are received by rt109, handled and sent back, but response is not received by rt53...

First I thought the problem is in MTU size, I changed it many times, but I discovered that allowed MTU size between my flats in provider's network is equal to 1500. Moreover, icmp packet is quite small one, much less than 1500 bytes.

This problem makes work of different services via tun0 very unattractive: sip-based voip works poor, file transfer is amazingly slow and so on. If you have some ideas, please, help me.